used to work at a dealership, when we had a trade in we would get some good degreaser and spray on the whole undercarriage and pressure wash off (a good self serve car wash will work, just wear some old clothes). Get everything off, let dry, and then use some flat black spray paint and go over the frame and axles, etc. Will make a big difference in the look, at least for a while. I used to use about 5-6 cans for a fullsize truck
Boyd